The present invention has for its subject a process for manufacturing printed circuits including through holes, comprising a cleaning pretreatment and a subsequent metallization treatment, wherein the cleaning pretreatment includes a possible treatment by organic solvent, a treatment in an alkaline solution of permanganate and a treatment for permanganate reduction; and the metallization treatment includes a treatment for conditioning the walls of the through holes, a treatment for mordanting the copper surfaces, a precatalysis treatment, a catalysis treatment and a metallization treatment by a chemical copper bath. The invention has also for its subject a composition for carrying out this process.
In the processes for manufacturing printed circuits it is required that the through holes are metallized in order that they become conductive for electrically connecting two or more printed faces of the circuit. The customarily used metallization process includes the electroless autocatalytic deposition of copper onto the walls of the through holes. Such process is known as the Plating-Through-Holes process (PTH). In order to deposit copper without intervention of electric current, the PTH metallization process is usually carried out by six main steps.
The first step of the PTH metallization process consists of a treatment by a surfactant degreasing and conditioning product, which has the purpose of degreasing the copper surfaces of the holed laminate by removing impressions, oily residues and the like, in order to allow a uniform etching during the subsequent mordanting treatment. Moreover this bath has the main purpose of conditioning the walls of the through holes, namely of suitably preparing them to the subsequent adsorption of catalytic particles. This function gives the name to the now considered step, which is known as the conditioning treatment. The conditioning consists in the neutralization of the negative electric charges present on the walls of the through holes, which hinder a uniform adsorption of the catalytic particles, these latter being negatively charged too.
The second step of the PTH metallization process consists of a treatment for mordanting the copper surfaces of the holed laminate, which is usually performed in a bath comprising inorganic acids, such as sulfuric acid of phosphoric acid, and stabilized hydrogen peroxide. As an alternative, solutions comprising alkaline persulfates are often used. This treatment has the purpose of uniformly roughening the copper surfaces of the laminates in order to allow a perfect adhesion of the subsequent depositions of chemical or electrolytic copper.
The third step of the PTH metallization process consists of a precatalysis treatment, usually performed in a bath containing sodium chloride or diluted hydrochloric acid, which has the purpose of protecting the subsequent catalysis bath from abnormal dilutions by water and from possible residues of copper or surfactant dragged from the foregoing baths. In effect, no intermediate rinsing is effected between the third and the fourth step.
The fourth step of the PTH metallization process consists of a catalysis treatment, which is usually performed in a bath comprising a solution based on palladium and tin chloride, acidified by hydrochloric acid. The catalysis bath allows depositing onto the walls of the through holes, suitably prepared by the foregoing treatments, a thin and uniform layer of a catalytic metal, which is capable of initiating, during the subsequent step of chemical copper plating, the electroless copper deposition.
The fifth step of the PTH metallization process consists of an acceleration treatment, which is usually performed by a solution based on inorganic acids such as fluoboric acid, hydrochloric acid, sulfuric acid and the like. The acceleration treatment has the purpose of limiting the palladium drag into the subsequent chemical copper bath, because an excessive drag of palladium could cause instability phenomena or even the complete decomposition of the chemical copper bath. Moreover, the acceleration treatment has the purpose of activating the catalytic layer deposited onto the walls of the through holes by removing the excess of tin chloride and then making the palladium extremely active. A more active catalytic layer promotes a complete covering of the walls of the through holes during the subsequent step of chemical copper deposition.
Finally, the sixth step of the PTH metallization process consists of a chemical copper deposition. During this step, copper is chemically deposited, without intervention of electric current, onto the suitably catalyzed walls of the through holes. After having completely covered with chemical copper the catalytic layer of palladium, the reaction proceeds autocatalytically by depositing copper until the desired thickness has been attained. In general, the chemical copper baths are formed by a solution of a copper salt in water, a copper complexing compound, a reducing agent and a pH regulator. These baths usually operate at a pH comprised between 11 and 13. In addition to these main components the chemical copper baths include several compounds which are present in low concentrations, such as stabilizers, surfactants and others. After the metallization in the chemical copper bath, the printed circuits may still receive further minor treatments, which are not considered here.
In manufacturing multilayer printed circuits, a specific pretreatment for cleaning the walls of the through holes, which is named "desmear", is usually needed before the PTH metallization process. This treatment has the purpose of eliminating possible residues deposited, during the boring operation, onto the walls of the through holes and particularly onto the inner copper layers. If such residues, which are formed by melted epoxy resin, are not accurately removed, during the subsequent metallization step the electric connection between the deposit on the hole wall and the inner copper layer cannot be ensured. When a more active cleaning treatment is performed onto the walls of the through holes, by attacking the epoxy resin up to a deep over 7,5 microns, such treatment is named "etch-back". In this case there is obtained a connection on three points of the chemical and electrolytic copper depositions onto the inner copper layers because, after the treatment, these latter project with-respect to the walls of the through holes.
Different methods are usually employed in order to perform the cleaning pretreatment onto the walls of the through holes. These methods are based on the use of chromic acid, concentrated sulfuric acid, an alkaline solution of permanganate, or even the plasma method is used. But among all these methods, the more employed is that based on an alkaline solution of permanganate. This method is generally performed in three steps.
The first step of the cleaning pretreatment (which in certain cases may be omitted) consists of a treatment in a bath based on an organic solvent, which has the purpose to soften the surfaces of the epoxy resin in order to allow, during the subsequent step of attack by an alkaline solution of permanganate, the obtainment of a micromordanted epoxy surface capable of successively ensuring a maximum adhesion of the chemical copper deposit. This bath may contain mixtures of organic solvents such as n-methyl-2-pyrrolidone, dimethylformamide, butylcarbitol, butylcellosolve, butylcellosolve acetate and the like, or alkaline solutions of these organic solvents in water.
The second step of the cleaning pretreatment consists in properly cleaning the walls of the through holes by a treatment in an alkaline solution based on permanganate. This solution is capable of attacking the residues left by the boring operation by an action of oxidative type, removing them from the walls of the through holes and particularly from the inner copper layers. After this treatment the walls of the through holes appear free from boring residues and very well prearranged for the subsequent chemical copper deposition. Usually these solutions are based on potassium or sodium permanganate and sodium hydroxide and they may comprise, in addition to a surfactant, a secondary oxidizer such as sodium persulfate or sodium hypochlorite. This secondary oxidizer has the purpose of keeping in the solution a high concentration of permanganate to the detriment of the manganate, which is the reduction product thereof.
The third step of the cleaning pretreatment on the walls of the through holes consists of a treatment in a neutralization bath, which performs the function of reducing the permanganate kept by the surface of the printed circuits and adsorbed on the walls of the through holes. During this step the residual permanganate is reduced in acidic ambient to manganese ion, which is soluble and therefore may easily be removed. Usually this reduction bath is based on sodium bisulfite, hydrazine, hydroxylamine or the like, and it may contain a surfactant too.
The pretreatment for cleaning the walls of the through holes before the metallization with chemical copper has been developed as a specific treatment for the multilayer printed circuit, and then it has been extended to the double face printed circuits. In effect, by this pretreatment it is possible to obtain a chemical copper metallization characterized by a high adhesion to the walls of the through holes.
Moreover by this pretreatment the metallization is performed onto a coherent surface, free from boring residues, and therefore the chemical copper deposit is dense and free from pores. The deposit obtained during the subsequent step of electrolytic copper deposition has a high density and a noticeable resistance to the subsequent thermic treatments. As a result, the printed circuits manufactured by performing a pretreatment for cleaning the through holes by means of permanganate before the chemical copper metallization may be perfectly soldered, they show a practically negligible percentage of through holes uncoupled or missing the ascended tin-lead alloy, and have very good properties of adhesion of the deposits on the walls of the through holes.
However the cleaning pretreatment on the through holes before the chemical copper metallization involves a noticeable increase in the manufacturing costs of the printed circuits, because to the cost of the proper treatment one should add the cost descending from the time needed for charging and discharging the printed circuits on frames or in baskets, once for the cleaning pretreatment of the through holes and then again for the PTH metallization process. -The additional cost of such a pretreatment is always accepted when it is applied to multilayer printed circuits, in view of the high value of the circuits of this kind, but not always the cost of this treatment may be accepted in the case of double face printed circuits, whose value is lower.
A method for reducing this cost resides in integrating the metallization process and the cleaning pretreatment by arranging a single line for cleaning and metallizing the through holes, in that the line needed for the cleaning pretreatment of the through holes is inserted immediately in front of the line for metallization by chemical copper. In this way there is no more the need for charging and discharging twice the frames of baskets. Such a method involves in all cases a high starting investment for the installation but, in addition thereto, many factories for the manufacture of printed circuits do not have available a physical space sufficient for inserting the cleaning line in front of the metallization line.
In order to simplify the process for the manufacture of printed circuits, some cycles for cleaning and metallizing the through holes have been proposed, wherein the number of needed treatments has been reduced. Two of these "short" cycles are described in the European Patent No. 0,261,424. Such short cycles are advantageous in that they reduce the overall number of treatments, but in many cases they are not sufficient for allowing to perform a metallization process integrated with a cleaning pretreatment, because many existing installations do not have available a sufficient number of tanks. In effect, as previously described, the conventional PTH metallization process requires six treatments, and the process for cleaning the through holes requires further three tanks, whereby in order to perform in a single line the complete process for cleaning and metallizing the through holes nine treatments are needed. The short cycles described in the European Patent No. 0,261,424 still require seven treatments, and therefore they cannot be directly performed in the conventional metallization lines, which are designed for performing only six treatments, with the corresponding intermediate rinsing operations. Therefore, in order to employ the new cycles, it is needed that some substantial modifications be made in the structure of the conventional metallization lines, by introducing new tanks and modifying the automated system for the transportation of the frames or baskets.